1. Field of the Invention
The present invention relates to pressure responsive diaphragm type control valves for controlling the flow of liquid. More particularly, the valve includes an independent diaphragm which is responsive solely to the differential pressure between the control pressure and the pressure of the controlled fluid that flows through the valve.
2. Description of the Prior Art
Diaphragm type control valves are used to control all kinds of fluids. Generally, the valve includes a diaphragm for supporting a valve element for movement into and out of engagement with a valve seat. A valve stem is connected to the valve element to guide the movement of the valve element into and out of the engagement with the valve seat. Springs are often mounted on the valve stem to aid in opening or closing of the valve element. The movement of the diaphragm is, therefore, restricted by the mass of the valve stem, its friction, and the mass of the valve element and the bias of the spring. Diaphragm type control valves of the type contemplated herein position the diaphragm in a spaced relation to a valve seat with the diaphragm being moved by hydraulic or pneumatic pressure to close on the valve seat.
It has been found that at high pressures the flat valve seat, which cooperates with the diaphragm to control the flow of fluid, imparts a Coanda effect to the flowing liquid on the face of the valve seat at higher pressures. As the velocity of liquid increases across the surface of the valve seat, a Coanda phenomenon is generated. The phenomenon is believed to be due to a combination of circumstances: the change of direction of the flow of liquid across the valve seat; the confinement of this flow to the face of the valve seat; and the higher operating pressure of the fluid, 30 psig and up. The phenomenon produces a pulsating movement of the diaphragm which shocks the valve and associated piping. It is believed that the fluid sucks the diaphragm toward the valve seat due to the acceleration of the fluid across the valve seat. This produces a pressure build up on the diaphragm which forces the diaphragm open and thus producing another accelerating pulse of fluid across the valve seat and another cycling of the diaphragm.